There are proposed methods of adjusting sound pressure levels of reproduced sound in an acoustic space such as a vehicle interior. For example, Patent Reference 1 discloses a method of cancelling peaks and dips of sound outputted by a plurality of speakers provided in a vehicle interior by using two or more speakers.
Patent Reference 2 discloses a method of automatically determining an equalizing filter characteristic for an indoor communication system of a cabin for a vehicle. Specifically, it is disclosed that a maximum value of magnitude of transfer function is determined to provide attenuation at a frequency band at the center of the maximum value.
Patent Reference 3 discloses a method of designing one or more filters to substantially equalize a frequency response in a frequency range for a listening area.
Patent Reference 4 discloses a method of setting an equalizer by reading out set values in accordance with presence or absence of a person at each passenger position in an interior of a movable body, in an acoustic control device for controlling sound signals outputted in the interior of the movable body.